This invention relates generally to a design of a page wide thermal ink jet print head and more particularly concerns a thermal ink jet print head which has a page wide monolithic channel plate with a plurality of heater chips bonded thereto. It is possible to make the long heater chips. However, usually the yield is much less than for smaller chips. Therefore, it is desirable to make a plurality of small heater chips, which have less chance of containing defective heating elements, and abut them together.
Traditionally channels are made on one chip and heating elements are made on a different chip and the two chips are bonded together to form a thermal ink jet module. Presently, an approach is used to abut several co-linear thermal ink jet modules to form a long print head. In this technology, it is extremely difficult to make a long array of modules.
The difficulty arises from bonding two wafers together. To avoid the alignment of each individual heater chip to each individual channel chip (if the heater chips and channel chip were diced from the wafers prior to assembly), the heater wafer is bonded to the channel wafer which requires only one alignment. Once the wafers are bonded together one can not tell where the space between the modules are and therefore, accurate cutting between the modules becomes extremely difficult. As a result, a four step cut is required to dice the modules. Two precise and shallow cuts, one along the edge of a channel chip and another along the edge of a heater chip, are made before bonding the two wafers together. The next two cuts are made after the two wafers are bonded together; one groove shaped cut from the top and another groove shaped cut from the bottom to get to the shallow cut. An imprecise cut causes the edges of the modules crack and the polyimide layers adjacent to the edges of the modules flake and as a result, the edges of the print head modules become jagged. Therefore, abutting the print head modules becomes difficult.
The next problem is aligning the modules before abutting them. The channels and nozzles of the abutted modules have to be on one plane while the pitch between the channels on the different modules where they are abutted must be the same as the pitch between the rest of the channels on the same module. Due to the imprecise cuts to dice the modules, large tolerances are introduced.
One approach to avoid the tolerance problems caused by imprecise cuts is to arrange the modules on two print head bars with staggered modules precisely placed. The two print head bars are placed in a machine in a precise distance apart. In this method, the modules are staggered on two bars in an alternate relationship. This technique requires a lot of mechanical precision and adds additional electronics to compensate for this construction.
Another approach as disclosed in U.S. Pat. No. 4,829,324, titled "Large array Thermal Ink Jet Print Head" is to have a page wide monolithic heater plate with abutted channel chips bonded to the heater plate. This technique, just like the aforementioned techniques, has the problem of aligning the channels and nozzles and keeping the pitch between the channels and nozzles on the different modules where they abut the same as the pitch between the rest of the channels and nozzles on the same module.